The present invention relates to an image forming apparatus that makes a cleaning toner image to be born on the surface of an image bearing element and then cleans the surface of the image bearing element by a cleaning element.
Some electrophotographic image forming apparatuses are configured to transfer a toner image that is born on an image bearing element, such as a photoreceptor drum and an intermediate transfer element, onto a sheet in a transfer position between the image bearing element and the transfer element. After the toner image is transferred onto the sheet, if residual toner which has remained on the surface of the image bearing element is not removed and the next image forming process is performed, the image quality of the image formed on the sheet will deteriorate, so that it is necessary to remove the residual toner by cleaning the surface of the image bearing element after the toner image is transferred.
When the image bearing element is being cleaned and the toner on the surface of the image bearing element is low, if the surface of the image bearing element contacts the cleaning element to be cleaned, friction will increase between the surface of the image bearing element and the cleaning element and will damage the surface of the image bearing element. In addition, when the toner on the surface of the image bearing element is high, if the surface of the image bearing element contacts the cleaning element to be cleaned, the toner will leak to the downstream side of the cleaning element in the moving direction of the surface of the image bearing element. Therefore, when the image bearing element is being cleaned, the predetermined amount of toner without excess or insufficiency needs to exist on the surface of the image bearing element. Accordingly, one conceivable approach is to supply cleaning toner so that the predetermined amount of toner may exist on the surface of the image bearing element before the surface of the image bearing element contacts the cleaning element.
For example, the image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 2006-251138 adjusts the amount of cleaning toner to be supplied according to the image density of the image data used for the immediately preceding image forming process in consideration that the amount of the residual toner that adheres to the surface of the image bearing element decreases as the image density of the image data used for the immediately preceding image forming process is lower.
It is to be noted that the image density in an electrophotographic image forming apparatus may preferably represent an image dot ratio that indicates a value obtained by dividing an area of an image part by an area of the sheet, the image part being converted from a number of dots of an ON signal in the image data; and is a concept included in print coverage. The print coverage is a concept that may preferably include an image area ratio as well as the image dot ratio, the image area ratio indicating a value obtained by dividing an integrated area of the image part of the sheet that has been subjected to an image forming process by the area of the sheet.
However, since the residual toner is accumulated on the surface of the image bearing element every time the image forming process is performed, even if the print coverage of the immediately preceding image forming process is low, the amount of toner that adheres to the surface of the image bearing element may increase. In addition, even if the print coverage of the immediately preceding image forming process is high, the amount of toner that adheres to the surface of the image bearing element may decrease. When the amount of the cleaning toner is adjusted only according to the print coverage in the immediately preceding image forming process and is supplied to the surface of the image bearing element, the toner that exists on the surface of the image bearing element might become excessive or insufficient.
In view of the foregoing, it is an object of the present invention to provide an image forming apparatus that can supply cleaning toner to the surface of an image bearing element without excess or insufficiency and clean the surface of the image bearing element by adjusting the amount of the cleaning toner according to the print coverage of a sheet in a predetermined period.